The invention relates to a design and arrangement of fuel for a nuclear reactor and to a method of operating such an arrangement.
Nuclear fuels include uranium and/or plutonium in suitable form. For example, in commonly used fuel for water cooled and moderated nuclear power reactors the fuel comprises uranium dioxide (UO.sub.2) in which from about 0.7 to 4.0 percent is fissile U-235 mixed with fertile U-238. During operation in the reactor some of the fertile U-238 is converted to fissile Pu-239 and Pu-241 which contributes to maintenance of reactor power. The U-238 also is fissile but only for high energy neutrons in the reactor.
In well-known commercial boiling water nuclear power reactors, for example as used in the Dresden Nuclear Power Station near Chicago, Illinois, the nuclear fuel typically is in the form of sintered pellets contained in an elongated cladding tube, formed of a suitable metal such as a zirconium alloy, to form a fuel element or rod as shown, for example, in U.S. Pat. No. 3,365,371. The tube, sealed by end plugs, serves to isolate the nuclear fuel from the moderator-coolant and to prevent the release of fission products.
Such fuel elements are arranged in groups and supported between upper and lower tie plates in separately replaceable fuel assemblies or bundles as shown, for example, in U.S. Pat. No. 3,689,358. A sufficient number of such fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a working fluid and a neutron moderator.
Nuclear reactors are typically refueled periodically with an excess of reactivity sufficient to maintain operation throughout an operating cycle often in the order of one year in length of time. The reactor is then shut down and a fraction of the fuel assemblies typically about one-quarter of the fuel assemblies, are replaced. The excess reactivity at the beginning of a cycle of operation requires a control system of sufficient strength to maintain the effective multiplication factor at unity during reactor operation. The control system customarily comprises neutron absorbing materials that serve to control the neutron population by nonfission absorption or capture of neutrons.
Typically, the control system includes mechanical control in the form of a plurality of control rods, containing neutron absorbing material, which are selectively insertable in the spaces or gaps among the fuel assemblies to control the reactivity, and hence the power level of operation, of the core. In a known arrangement, such as shown for example in U.S. Pat. No. 3,020,888, the control rod blades have a cross or cruciform transverse cross section whereby the "wings" of the blades of each control rod are insertable in the spaces between an adjacent four fuel assemblies. Each such cluster of four fuel assemblies surrounding a control rod can be called a core "cell". (Suitable neutron absorbing materials and control blade drive mechanism are set forth in the above-mentioned U.S. Pat. No. 3,020,888.)
The control system may also include a burnable neutron absorber, such as gadolinium, blended with some of the fuel. The naturally occurring Gd-155 and Gd-157 isotopes are strong neutron absorbers which are converted by neutron absorption to isotopes of lesser control worth (neutron absorbing capacity). Such use of burnable absorbers decreases the amount of mechanical control required and, by appropriate arrangement of the burnable absorber, improvements in power distribution can be achieved. Frequently such burnable absorbers are incorporated in the fuel elements in a mixture with selected portions of the nuclear fuel. An arrangement of burnable absorber is shown, for example, in U.S. Pat. No. 3,799,839.
Further information on nuclear reactors may be found, for example in "Nuclear Power Engineering", N. M. El-Wakil, McGraw-Hill Book Company, Inc., 1962.
The tubular fuel element cladding, which may be in the order of 0.032 inches (0.8 mm) in thickness, is subjected to relatively severe service because of the high pressure, high temperature, nuclear radiation and chemical fission product attack in the environment of the nuclear reactor core. Withdrawal of inserted control rods sharply increases local power in adjacent fuel elements. Such sudden large changes in the local linear power level (kw/ft) of fuel can cause high local stresses and strains from interaction of the fuel pellets with the cladding. If the expanding, separating edges of adjacent pellets (or adjacent sides of a pellet crack) lock against the cladding, the resulting localized strain may exceed the ultimate strain of the cladding and cause cracks therein which permit entry of coolant into the fuel element and escape of fission products from the fuel element into the surrounding coolant. This undesirable phenomenon has become known as "pellet-cladding interaction." There is a fuel burnup dependent threshold below which the cladding is known not to fail, independent of the magnitude of increase in linear power.
Among the proposed solutions to the pellet-cladding interaction (PCI) problem a method for conditioning the fuel to withstand subsequent rapid power changes has come into use. Such method is described in U.S. Pat. No. 4,057,466. Briefly, the method comprises a regulated, systematic control of the rate of power increase (e.g., less than 0.1 kw/ft/hr) to permit the local PCI producing pellet forces to relax. The rate is controlled below the critical rate which causes cladding damage for increases in local linear power between the PCI threshold and the desired maximum local linear power level. After such conditioning it is found that relatively rapid power changes, below the maximum conditioned level, can be made without cladding damage. The primary disadvantage of the method is the relatively lengthy time required for such conditioning or reconditioning since it decreases the time available for operation at normal power levels. Also, in many practical operating situations it is impossible to fully condition fuel near inserted control rods.
Another phenomenon attendant the operation of a nuclear reactor is called "control rod history." The effect of the presence of a control rod blade is to reduce greatly the rate of fissile fuel burn-up in the adjacent fuel whereas the conversion of fertile U-238 to fissile Pu-239 is reduced to a significantly lesser extent. Thus when a control rod is withdrawn, the power in the "uncovered" fuel nearest to the control rod (i.e., in the corner and adjacent peripheral fuel elements of the fuel assembly) increases to a greater extent than in the fuel further removed from the control rod. This control rod history effect is greatest for the fuel in the corner fuel element of the fuel assembly adjacent the control rod and the effect becomes more pronounced the longer the control rod remains adjacent the fuel. The control rod history effect is greatest for designs in which there are no followers on the control rods.
Another phenomenon prevalent in boiling water reactors is "axial steam void suppression." In such reactors, boiling of the coolant within individual channels causes a negative power feedback because the local reactivity of the fuel decreases with increasing steam voids. If a control rod is partially inserted in the bottom of a channel it suppresses the boiling near the control blade thereby causing a corresponding reduction in steam voids in the higher reactivity regions above the control blade. The reduced boiling above a partly inserted control rod can cause severe power peaking which can exceed the magnitude of the power in a channel with the control rod flully withdrawn.
Early in the design and operation of nuclear power reactors of the type under discussion, procedures and patterns for control rod insertion and withdrawal were developed. The basic approach has been to attempt to distribute the fuel burn-up, plutonium production and control rod history effects as evenly as possible among the fuel assemblies of the core by periodic revision and interchange (swapping) of control rod patterns.
In known control rod operating procedures, for reactors of the type described, the control rods are arranged in several alternating patterns which permit one group of control rods to be swapped for another during operation. These usually consist of two, three or four patterns of control rods which alternatively are inserted in the reactor core for power shape and burnup reactivity control.
In accordance with the known control rod operating procedures the core is operated with a given control rod pattern for a period of energy generation. Power is then reduced and the given pattern is exchanged or swapped for another pattern, etc. Thus, there may be from five to eight control rod pattern changes in an annual reactor operating cycle. Such control rod patterns and pattern swapping are discussed in greater detail in U.S. Pat. No. 3,385,758.
The known control rod operating procedures cause most of the fuel to experience adjacent control rod movement at power during the residence time of the fuel in the core of about four years. Such control rod motion results from burn-up control, control rod pattern swaps, load following, xenon transient control, fuel conditioning, etc. These operating variables cause the total number of control rod movements which the fuel experiences to be undesirably large. Furthermore, control rod pattern swaps excite spatial power distribution xenon transients and control rod motion constraints due to thermal, hydraulic, safety and fuel conditioning limits make reactor operation undesirably complex and increase the probability of operator error. Thus, the known operating procedures tend to decrease thermal and safety margins, increase manufacturing complexity, decrease capacity factors and increase the risk of fuel damage.
A summary of the observed problems with the known BWR operating methods which interchange or swap control rod patterns is as follows:
(1) The reactor power must be reduced to perform the swaps while conforming to PCI constraints. In many cases as many as five days are required to return the reactor to full power after the swap, thus reducing reactor capacity factor. PA1 (2) The swaps complicate design and operations. Because of the desirability of performing control rod swaps when power is reduced for some other reason, reactor operations are difficult to plan and similar reactors are operated differently through a fuel cycle interval. PA1 (3) The control rod swaps and associated power reductions cause spatial and non-spatial xenon transients which complicate reactor operations and contribute to difficulty in conforming to PCI limits. PA1 (4) The reactor operator learning curve is extended by the complicated, interacting three space dimensional variables and constraints. This increases the possibility of operator errors which would violate PCI or other constraints. PA1 (5) All fuel except that located at the periphery of the core experiences large increases in linear power due to adjacent control rod motion during the fuel cycle interval between refuelings. The core periphery usually is the only area where fuel can be located such that it will not experience adjacent control rod motion. PA1 (6) All control rods, except those located near the core periphery, must serve the dual functions of power shaping-burnup reactivity control and reactor shutdown. Therefore, any unique specialized design characteristics required for these different functions cannot easily be incorporated in the control rod and control rod drive designs. PA1 (7) Application of automatic power distribution spatial shape control for load following or other purposes is greatly complicated by the large number of variables and their complex interaction. Furthermore, the following disadvantages from locating control rods adjacent to high power or high reactivity fuel or adjacent to fuel containing undepleted burnable absorber have been observed: PA1 Low steam void water above partially inserted control rods frequently cause the peak local reactor power to be reached in such regions. Thermal limits such as the transition or departure from nucleate boiling limits also are affected adversely by partial control rod insertion adjacent to high reactivity fuel.
Control rods inserted adjacent to fuel containing undepleted burnable absorber tend to skew the burnup of the absorber and causes an undesirable burnable absorber spatial transient that increases peak local power in the reactor or requires that the fuel be designed and fabricated with complicated burnable absorber shapes.
It is an object of this invention to provide a fuel design, fuel arrangement and reactor operating methods which simplify reactor core operation. More specific objects of the invention are to minimize control rod movement throughout an operating cycle, maintain low reactivity fuel adjacent to inserted control rods, separate the control rod functions into power shaping-burnup control and shutdown functions, increase reactor capacity factors, increase fuel reliability, improve fuel cycle economics, increase thermal margins, increase the feasibility of automatic spatial power shape control and improve load following capability.